Contrasting Mo–U enrichments of the basal Datangpo Formation in South China: Implications for the Cryogenian interglacial ocean redox

Abstract Paleoredox reconstructions of Neoproterozoic successions have been the subject of long-standing debates, especially concerning the precise timing of ocean ventilation and its causal relationship, if any, with the evolution of early animal life. Sponge biomarkers and putative sponge-like body fossils have been found from rocks in the Cryogenian Period, yet the contemporaneous deep-water conditions remain poorly understood. To fill this gap, we report pyrite morphology, trace metal enrichments, and total organic carbon data from interglacial black shales of the lower Datangpo Formation at Daotuo area, northeast Guizhou Province, South China. A contrasting Mo–U enrichments pattern emerges from the basal Datangpo black shales, despite the persistently euxinic local environment as testified by the presence of abundant tiny pyrite framboids. Molybdenum is markedly enriched in the basal interval and its concentration decreases sharply up section, while uranium stays relatively constant at crustal concentrations throughout the entire profile. This decoupled Mo–U signature, integrated with a simple mass balance model, suggests that euxinia only covered a small proportion of the dominantly ferruginous global seafloor, thus allowing dissolved Mo to accumulate to significant level in seawater. The enrichment degrees of Mo, although nontrivial, are still much lower than those from Phanerozoic equivalents. Contrarily, they are quite close to the total mean abundance for Mesoproterozoic euxinic shales. Such a mild increase of marine Mo–U inventory is therefore inferred as recording no dramatic rise in oxygen levels following the Sturtian glaciation. A drop of Mo contents after the basal interval could represent either limited Mo availability or inadequate hydrogen sulfide supply.